The invention relates to a system which can decrease the pressure in a pressurized water nuclear reactor so that additional coolant can be added at low pressure. More particularly, the invention provides a system of valve connections to spargers opening flowpaths between a hot leg of the primary reactor coolant circuit and the in-containment refueling water storage tank. The valve system is operable in stages for quickly reducing the pressure without sudden hydraulic loading of the respective reactor conduits.
Depressurization systems comprising sparger vents are used in boiling water reactors, for over-pressure protection and so that coolant can be added using a low pressure pump rather than a high pressure pump. When commencing depressurization, one or more valves coupling the coolant circuit to spargers are simply opened. The spargers comprise conduits with small jet orifices submerged in a tank of water, for example at atmospheric pressure. When the valve(s) are opened, steam is emitted from the sparger orifices into the tank, and coolant condenses in the water, with consequent reduction of pressure in the coolant circuit.
In conventional depressurization of a boiling water reactor, the object is not to reduce the coolant pressure to atmospheric pressure, but only to reduce the pressure to the point where pumps are effective to inject water into the coolant circuit. The spargers are intended to reduce the pressure to around 100 to 200 psi, to allow injection of water from a relatively low pressure pump.
A pressurized water reactor operates at substantial coolant pressure. The temperature of the coolant in a pressurized water reactor having even a modest power level may be on the order of 600.degree. F. (330.degree. C.). The operational coolant pressure in the reactor vessel may be 2,250 psi (150 bar).
It may be desirable to add water to the coolant circuit of a reactor in various circumstances. When operational, the level of coolant may be lowered slightly due to leakage through designed means such as pressure reliefs, or due to leakage through a small opening in the coolant circuit which does not represent a critical defect, eventually requiring that coolant be added. In a severe loss of coolant accident, a substantial break in the coolant circuit can flush a large quantity of coolant into the containment shell. A depressurization system should respond appropriately to any eventuality, enabling sufficient coolant to be added to comply with the ultimate requirement to cool the nuclear fuel.
Where the connections between a coolant circuit or tank at high pressure and a low pressure outlet such as a sparger are made suddenly, a severe thermal and hydraulic load is placed on the conduits and on the coolant circuit as a whole. Moreover, the initial coupling of the coolant circuit with the pressure relieving spargers results in a substantial loss of coolant driven by the very high pressure differential between the coolant in the circuit and the outlets (i.e., the sparger orifices). For both these reasons, there is a potential for damage to the reactor during depressurization. Apart from the possibility of causing faults in the conduits due to thermal and hydraulic stress, the loss of coolant results in an increased danger that the remaining coolant may be insufficient to maintain cooling of the nuclear fuel. There is a need for a depressurization system which minimizes stress and conserves coolant, by a more gradual opening of flow to the pressure relief.
It is desirable in a nuclear reactor safety system also to minimize reliance on active elements such as pumps. U.S. Pat. No. 4,753,771 --Conway et al discloses a safety system which employs high pressure and low pressure supplies of makeup water for addition to the coolant. The low pressure supply has a tank at atmospheric pressure (i.e., at the pressure within the containment shell) and is arranged in the containment at an elevation above the coolant circuit, coupled to the coolant circuit via a check valve. A high pressure makeup tank provides water for a short time in the event of a leak, without depressurization. Some time during the depletion of the high pressure water supply, it is necessary to depressurize the reactor coolant system to allow the much larger amount of water available from the low pressure supply to be added. This addition occurs at a relatively low positive pressure (due to the fluid pressure head of the storage tank) or even at atmospheric pressure.
According to the Westinghouse Electric Corporation AP600 reactor design, of which the present invention is a part, a high pressure makeup tank is provided with about 20 minutes of water supply. After depressurization of the coolant circuit in an emergency situation, a gravity drain tank at atmospheric pressure in the containment shell provides approximately ten hours of water supply. When the gravity drain tank is emptied into the containment shell, the containment is filled to a point where recirculation of water condensing on inner walls of the containment becomes possible. The gravity tank holds sufficient water that when the tank empties into the containment, the height of the water in the containment provides a sufficient fluid pressure head to force water into the reactor (located low in the containment). The water boils in the reactor and thereby cools the core. The depressurization system vents the steam from the boiling water into the containment shell. The steam condenses on the inner walls of the containment and returns as water to cool the reactor.
The depressurization occurs in stages. Initially, relatively smaller conduits leading to submerged spargers are opened by valves. Relatively larger conduits are then opened by further valves, the larger conduits also leading to spargers. Ultimately, in a final stage, the coolant system is vented directly into the containment.